This invention relates to acoustic imaging in general and more specifically to a method and apparatus for detecting internal structures in bulk objects using acoustic imaging.
Various types of acoustic imaging processes have been developed over the years in which an acoustic wave is used to collect information relating to certain internal features and structures in objects. Acoustic imaging processes are useful for this purpose since acoustic waves pass easily through most objects.
While some types of acoustic imaging processes utilize only the amplitude of the acoustic wave in order to detect certain internal features and structures of the object, it is known that considerably more information may be obtained about such internal features if both the phase and the amplitude of the acoustic wave are captured. Consequently, many acoustic imaging processes have been developed which capture both the phase and amplitude of the acoustic wave after it has traveled through the object. Once the phase and amplitude have been captured, any of a wide variety of processing methods and/or wave reconstruction techniques may be used to produce an xe2x80x9cacousticxe2x80x9d image of the object. Depending on the particular processing method that is used, the resulting acoustic image may reveal certain interior features, structures, and/or faults which may be contained within the object.
While systems for producing such acoustic images are known and have been used, such systems are not without their problems. For example, one significant problem that heretofore has imposed significant limitations on acoustic imaging processes relates to capturing the phase and amplitude of the acoustic wave. Any errors or distortions that may arise from or be introduced in the acoustic wave capturing process will adversely affect the resulting acoustic image data unless suitable processes or methods are employed to compensate for any such errors or distortions.
For example, in one type of acoustic imaging system, a two-dimensional array of microphones is used to capture and record both the phase and amplitude of the acoustic wave emanating from the ensonified object. Unfortunately, however, the physical size of each microphone prevents a two-dimensional array of such microphones from collecting little more than a relatively coarse, sampling of the phase and amplitude of the acoustic wave. Such coarse sampling limits the spatial resolution available with such a process. Another problem with such microphone systems is that the microphones are typically located a spaced-distance from the surface of the ensonified object. The intermediate medium (e.g., air) located between the object surface and the microphones distorts the acoustic data which, if not fully and correctly removed or compensated, distorts the final acoustic image data. The removal or compensation of such distortions is by no means trivial, and to date, no system has been developed that fully and completely compensates for the distortion arising from the passage of the acoustic wave through the intermediate medium (e.g., air). Moreover, the presence of air between the surface of the ensonified object and the microphones also tends to limit the sensitivity of the acoustic imaging system.
Partly in an effort to solve the spatial resolution problem associated with the use of a two-dimensional array of microphones, acoustic imaging processes have been developed in which the two-dimensional array of microphones is replaced by a single microphone. This single microphone is then scanned in two-dimensions in a raster-like manner in order to record the phase and amplitude of the acoustic wave emanating from the ensonified object. While the single microphone scanning system allows for increased spatial resolution, it is limited in temporal resolution in that it requires a finite time to move the scanning microphone over the desired data collection area. Consequently, such scanning type systems are only useful if the acoustic wave pattern is essentially time invariant during the period required to complete the scan. That is, if the acoustic wave pattern changes during the scan, the resulting acoustic image data will be distorted. Of course, such single microphone systems are still prone to the distortion problems resulting from the fact that the acoustic waves travel through the intermediate medium (e.g, air) before reaching the microphone.
Another class of acoustic imaging systems, generally referred to as immersion type systems, require that the object to be studied be immersed in a liquid, such as water. The liquid acts as an acoustical amplifier, thereby increasing the sensitivity of the acoustic imaging system over systems in which air comprises the intermediate medium. In one type of immersion process, a suitable object acoustic wave generator is placed in the liquid along with the object and is used to ensonify the object. This results in the production of an object wave which eventually reaches the surface of the liquid. The object wave is then combined with a reference acoustic wave which is produced by a separate reference wave generator that is also submerged within the liquid. The reference and object waves combine and interfere with one another, resulting interference pattern on the surface of the liquid. The interference pattern forms a diffraction grating that is capable of diffracting light. The surface of the liquid is then illuminated by a coherent light source, such as a laser beam, which is thereafter diffracted by the acoustic wave interference pattern on the liquid surface. The diffracted light beam is then combined with a reference light beam to form an optical hologram that is related to the acoustic wave contained on the surface of the object. The information contained in the optical hologram of the liquid surface may be used to extract the phase and amplitude information of the acoustic wave.
One advantage of the immersion system described above is that:it does not experience the same spatial or temporal resolution problems that are typically associated with the microphone systems described above, since the optical hologram of the liquid surface may be resolved to very high resolutions. Unfortunately, however, such immersion systems are still prone to the difficulties associated with the intermediate medium (e.g., the liquid) located between the ensonified object and the xe2x80x9cdetectorxe2x80x9d (e.g., the surface of the liquid). Here again, while methods have been developed which can partially compensate for the distortions produced by the intermediate medium, the correction methods are not complete and still result in acoustic image data having a considerable degree of distortion. Of course, another disadvantage associated with such liquid immersion methods is the requirement that the object be submerged in the liquid medium.
Consequently, a need remains for an acoustic imaging system that is capable of recording both the phase and amplitude information of the acoustic wave, but is not subject to the problems and limitations associated with prior art systems. Additional advantages could be realized if such an acoustic imaging system eliminated the need for is an intermediate medium (e.g., air or a liquid) in order to carry or transmit the acoustic wave from the ensonified object to the detector system.
Apparatus for producing an acoustic image of an object according to the present invention may comprise an excitation source for vibrating the object to produce at least one acoustic wave therein. The acoustic wave produced in the object results in the formation of at least one surface displacement on the surface of the object. A light source produces an optical object wavefront and an optical reference wavefront and directs the optical object wavefront toward the surface of the object. The interaction of the optical object wavefront with the surface displacement on the object produces a modulated optical object wavefront. A modulator operatively associated with the optical reference wavefront modulates the optical reference wavefront in synchronization with the acoustic wave to produce a modulated optical reference wavefront. A sensing medium positioned to receive the modulated optical object wavefront and the modulated optical reference wavefront combines the modulated optical object and reference wavefronts to produce an image related to the surface displacement on the surface of the object. A detector detects the image related to the surface displacement produced by the sensing medium. A processing system operatively associated with the detector constructs an acoustic image of interior features of the object based on the phase and amplitude of the surface displacement on the surface of the object.
Also disclosed is a method for detecting an internal structure of an object that is comprises the steps of: Vibrating the object to produce at least one acoustic wave therein; directing an optical object wavefront toward the surface of the object so that the optical object wavefront is modulated by the surface displacement on the object to produce a modulated optical object wavefront; modulating an optical reference wavefront in synchronization with the vibrating object to produce a modulated optical reference wavefront so that a difference frequency between the modulated optical object wavefront and the modulated optical reference wavefront is within the response range of a sensing medium; combining the modulated optical object wavefront and the modulated optical reference wavefront within the sensing medium to produce an image of the surface displacement on the object; and detecting the internal feature of the object based on the image of the surface displacement.